A THERMODYNAMIC NON-EQUILIBRIUM SLUG FLOW MODEL EXPLAINS ENHANCEMENT OF BOILING HEAT TRANSFER IN WATER AT LOW PRESSURES
Heat transfer peaks at the vicinity of the point of near-zero equilibrium quality were observed experimentally by a number of in-tube flow boiling investigators. The occurrence of such peaks is typical of systems where the rate of vapour formation for a given heat flux is large (e.g., low-pressure water and hydrocarbons). Previous work showed that a possible mechanism for this heat transfer enhancement is the existence of thermodynamic non-equilibrium slug flow, i.e., a type of flow in which sub-cooled slugs of liquid are separated by saturated Taylor bubbles that grow rapidly as a result of vigorous vapour formation.
In the present work, a model is advanced and applied to the problem of in-tube forced convective boiling of water at low pressures. The experimental database of Cheah (Ph.D. thesis, Imperial College, 1996) for water undergoing forced convective vaporization in a vertical (23 mm ID) channel was used to validate the model. The heat transfer coefficient trends as a function of increasing quality (in the region of low quality) were predicted both qualitatively and quantitatively by the methodology, which represents an improvement over existing correlations.